Apparatus and methods for multicomponent marine geophysical data gathering

ABSTRACT

In one embodiment the invention comprises a particle velocity sensor that includes a housing with a geophone mounted in the housing. A fluid that substantially surrounds the geophone is included within the housing. The particle velocity sensor has an acoustic impedance within the range of about 750,000 Newton seconds per cubic meter (Ns/m 3 ) to about 3,000,000 Newton seconds per cubic meter (Ns/m 3 ). In another embodiment the invention comprises method of geophysical exploration in which a seismic signal is generated in a body of water and detected with a plurality of co-located particle velocity sensors and pressure gradient sensors positioned within a seismic cable. The output signal of either or both of the particle velocity sensors or the pressure gradient sensors is modified to substantially equalize the output signals from the particle velocity sensors and the pressure gradient sensors. The output signals from particle velocity sensors and pressure gradient sensors are then combined.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is related to marine geophysicalexploration. More specifically, the invention is related to sensors fordetecting seismic signals and to marine seismic data gathering.

[0003] 2. Description of Relevant Art

[0004] In seismic exploration, geophysical data are obtained by applyingacoustic energy to the earth at the surface and detecting seismic energyreflected from interfaces between different layers in subsurfaceformations. The seismic wave is reflected when there is a difference inimpedance between the layer above the interface and the layer below theinterface.

[0005] In marine seismic exploration, a seismic shock generator, such asan airgun, for example, is commonly used to generate an acoustic pulse.The resulting seismic wave is reflected back from subsurface interfacesand detected by sensors deployed in the water or on the water bottom.

[0006] In a typical marine seismic operation, a streamer cable is towedbehind an exploration vessel at a water depth between about six to aboutnine meters. Hydrophones are included in the streamer cable fordetecting seismic signals. A hydrophone is a submersible pressuregradient sensor that converts pressure waves into electrical signalsthat are typically recorded for signal processing, and evaluated toestimate characteristics of the earth's subsurface.

[0007] After the reflected wave reaches the streamer cable, the wavecontinues to propagate to the water/air interface at the water surface,from which the wave is reflected downwardly, and is again detected bythe hydrophones in the streamer cable. The reflection coefficient at thesurface is nearly unity in magnitude and negative in sign. The seismicwave will be phase-shifted 180 degrees. The downwardly traveling wave iscommonly referred to as the “ghost” signal, and the presence of thisghost reflection creates a spectral notch in the detected signal.Because of the spectral notch, some frequencies in the detected signalare amplified and some frequencies are attenuated.

[0008] Because of the ghost reflection, the water surface acts like afilter, making it difficult to record data outside a selected bandwidthwithout excessive attenuation or notches in the recorded data.

[0009] Maximum attenuation will occur at frequencies for which thedistance between the detecting hydrophone and the water surface is equalto one-half wavelength. Maximum amplification will occur at frequenciesfor which the distance between the detecting hydrophone and the watersurface is one-quarter wavelength. The wavelength of the acoustic waveis equal to the velocity divided by the frequency, and the velocity ofan acoustic wave in water is about 1500 meters per second. Accordinglythe location in the frequency spectrum of the resulting spectral notchis readily determinable. For example, for a streamer water depth of 7meters, as illustrated by curve 54 in FIG. 1, maximum attenuation willoccur at a frequency of about 107 Hz. and maximum amplification willoccur at a frequency of about 54 Hz.

[0010] It has not been practical to tow cables deeper than about 9meters because the location of the spectral notch in the frequencyspectrum of the signal detected by a hydrophone substantially diminishesthe utility of the recorded data. It has also not been practical to towcables at a depth shallower than about 6 meters, because the ghostsignal reflected from the water surface substantially attenuates thesignal detected by a hydrophone within the frequency band of interest.

[0011] It is also common to perform marine seismic operations in whichin which sensors are deployed on the water bottom. Such operations aretypically referred to as “ocean bottom seismic” operations. In oceanbottom seismic operations, both hydrophones and geophones are employedfor recording the seismic data, with the geophone normally being placedin direct contact with the ocean bottom. To improve the contact betweenthe geophone and the ocean floor, the geophone assembly is typicallymade to be quite heavy, with a typical density of between 3 and 7 gramsper cubic centimeter.

[0012] A geophone detects a particle velocity signal, whereas thehydrophone detects a pressure gradient signal. The geophone hasdirectional sensitivity, whereas the hydrophone does not. Accordingly,the upgoing wavefield signals detected by the geophone and thehydrophone will be in phase, but the downgoing wavefield signalsdetected by the geophone and the hydrophone will be 180 degrees out ofphase. Various techniques have been proposed for using this phasedifference to reduce the spectral notch caused by the ghost reflection.

[0013] U.S. Pat. No. 4,486,865 to Ruehle, for example, teaches a systemsaid to suppress ghost reflections by combining the outputs of pressureand velocity detectors. The detectors are paired, one pressure detectorand one velocity detector in each pair. A filter is said to change thefrequency content of at least one of the detectors so that the ghostreflections cancel when the outputs are combined.

[0014] U.S. Pat. No. 5,621,700 to Moldovenu also teaches using at leastone sensor pair comprising a pressure sensor and a velocity sensor in anocean bottom cable in a method for attenuating ghosts and water layerreverberations.

[0015] U.S. Pat. No. 4,935,903 to Sanders et al. teaches a marineseismic reflection prospecting system that detects seismic wavestraveling in water by pressure sensor-particle velocity sensor pairs(e.g., hydrophone-geophone pairs) or alternatively vertically-spacedpressure sensors. Instead of filtering to eliminate ghost reflectiondata, the system calls for enhancing primary reflection data for use inpre-stack processing by adding the ghost data.

[0016] U.S. Pat. No. 4,979,150 provides a method for marine seismicprospecting said to attenuate coherent noise resulting from water columnreverberation by applying a scale factor to the output of a pressuretransducer and a particle velocity transducer positioned substantiallyadjacent one another in the water. In this method, the transducers maybe positioned either on the ocean bottom or at a location in the waterabove the bottom, although the ocean bottom is said to be preferred.

[0017] Four component system have also been utilized on the sea floor, Afour component system utilizes a hydrophone for detecting a pressuresignal, together with a three-component geophone for detecting particlevelocity signals in three orthogonal directions: vertical, in-line andcross line. The vertical geophone output signal is used in conjunctionwith the hydrophone signal to compensate for the surface reflection. Thethree orthogonally positioned geophones are used for detecting shearwaves, including the propagation direction of the shear waves.

[0018] The utility of simultaneously recording pressure and verticalparticle motion in marine seismic operations has long been recognized.However, a geophone (or accelerometer) for measuring vertical particlemotion must be maintained in a proper orientation in order to accuratelydetect the signal. Maintaining such orientation is non-trivial in amarine streamer and significantly more problematic than maintaining suchorientation on the ocean bottom. Exploration streamers towed behindmarine vessels are typically over one mile in length. Modern marineseismic streamers may use more than 10,000 transducers. To maintain aparticle velocity sensor (a geophone or accelerometer) in properorientation to detect vertical motion, the prior art has proposedvarious solutions. The use of gimbals has been proposed repeatedly. Oneexample is a “gimbal lock system for seismic sensors” described in U.S.Pat. No. 6,061,302 to Brink et al. Another example is a “dual gimbalgeophone” described in U.S. Pat. No. 5,475,652 to McNeel et al. Stillanother example is a “self-orienting directionally sensitive geophone”described in U.S. Pat. No. 4,618,949 to Lister. Nevertheless, nostreamers containing both hydrophone and geophones are in commercialuse.

[0019] In addition to the problem of maintaining orientation, severenoise from streamer cables has been considered prohibitive to use ofparticle velocity sensors in streamers. Because the voltage outputsignal from particle velocity sensors is normally not as strong as theoutput signal from hydrophones, the noise level in streamer cables hasbeen a detriment to the use of particle velocity sensors.

[0020] In ocean bottom cables, the sensors are located on the sea floorand therefore are less exposed to noise generated by vibrations in thecable. Geophones are typically gimbaled to ensure a correct directionand are made of heavy brass or similar material to ensure good contactwith the sea floor. The geophone housing is typically filled with fluidto improve the coupling between the sensor and the seafloor. However,because of the variation in properties of the seafloor from location tolocation, impedance mismatch between the seafloor and the sensor andsensor housing can cause problems. Such mismatch in impedance can causevarious types of distortion in both the hydrophone signal and thegeophone signal. Also, the boundary effects for the hydrophone and thegeophone due to their closeness to the sea floor can change the responsefor the hydrophone and the geophone, giving rise to a need to correctthe amplitude values in processing to be able to use the signal forelimination of the surface “ghost” reflection.

[0021] Accordingly a need continues to exist for an improved system forgathering marine seismic data.

SUMMARY OF THE INVENTION

[0022] In one embodiment the invention comprises a particle velocitysensor that includes a housing with a geophone mounted in the housing. Afluid that substantially surrounds the geophone is included within thehousing. The particle velocity sensor has an acoustic impedance withinthe range of about 750,000 Newton seconds per cubic meter (Ns/m³) toabout 3,000,000 Newton seconds per cubic meter (Ns/m³).

[0023] In another embodiment the invention comprises method ofgeophysical exploration in which a seismic signal is generated in a bodyof water and detected with a plurality of co-located particle velocitysensors and pressure gradient sensors positioned within a seismic cabledeployed in the body of water. The output signal of either or both ofthe particle velocity sensors or the pressure gradient sensors ismodified to substantially equalize the output signals from the particlevelocity sensors and the pressure gradient sensors within at least aselected frequency range. The output signals from co-located particlevelocity sensors and pressure gradient sensors are then combined.

[0024] In yet another embodiment the invention comprises a method ofprocessing marine seismic data to reduce spectral notches resulting fromsurface ghost reflections in which the amplitude and phase variationwith frequency of the output of a particle velocity sensor of aco-located particle velocity sensor and pressure gradient sensor pair isdetermined independently of any variation in amplitude or phase withfrequency of the particle velocity sensor output resulting fromimpedance mismatch between the particle velocity sensor and a mediumfrom which a seismic wave is coupled to the particle velocity sensor.The output signal of one or both of the particle velocity sensors orpressure gradient sensors is modified to compensate for the determinedamplitude and phase variation to generate modified output signals. Themodified output signals are then summed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 shows the frequency spectrum of a seismic signal detectedby a hydrophone at a water depth of 7 meters.

[0026]FIG. 2 illustrates a typical implementation of the invention, inwhich a plurality of streamer cables are towed behind a seismic surveyvessel.

[0027]FIG. 3 shows the geophone assembly with the parts exploded orseparated out for illustration.

[0028]FIG. 4 shows a cross section of a geophone assembly.

[0029]FIG. 5 shows particle velocity sensors and pressure gradientsensors in a seismic streamer cable.

[0030]FIGS. 6A and 6B show a typical phase and amplitude response for aparticle velocity sensor.

[0031]FIG. 7 shows the simulated output responses for a hydrophone and ageophone at a water depth of 26 meters.

[0032]FIG. 8 provides actual hydrophone and geophone data from a fieldtest with the cable at about 26 meters.

[0033]FIG. 9 shows a summation of the hydrophone and geophone data shownin FIG. 8.

[0034]FIG. 10 shows a simulation of streamer data at a one-meter depth.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0035]FIG. 2 illustrates a typical geophysical exploration configurationin which a plurality of streamer cables 30 are towed behind vessel 32.One or more seismic sources 34 are also normally towed behind thevessel. The seismic source, which typically is an airgun, but may alsobe a water gun or other type of source known to those of ordinary skillin the art, transmits seismic energy or waves into the earth and thewaves are reflected back by reflectors in the earth and recorded bysensors in the streamers. Paravanes 35 are utilized to maintain thecables 30 in the desired lateral position. The invention may also beimplemented, however, in seismic cables that are maintained at asubstantially stationary position in a body of water, either floating ata selected depth or lying on the bottom of the body of water, in whichcase the source may be towed behind a vessel to generate shock waves atvarying locations, or the source may also be maintained in a stationaryposition. Seismic sensors, in accordance with embodiments of the presentinvention are deployed in streamer cables, such as cables 30.

[0036] In a particular implementation, the present invention comprises aparticle velocity sensor in the form of a geophone assembly. Such ageophone assembly is shown in FIGS. 3 and 4. FIG. 3 shows the geophoneassembly 3 with the parts exploded or separated out for illustration.FIG. 4 shows a cross section of the geophone assembly 3 of FIG. 3 withthe various parts assembled (not exploded).

[0037] With reference to FIGS. 3 and 4, geophone 10 is mounted in ahousing 20 comprising outer sleeve 12 and end cups 1 and 13. Geophone 10is secured in mounting ring (or cradle) 8. Shafts 9 extend from oppositesides of mounting ring 10 into bushings 2. Bearings 4, which arepositioned between shafts 9 and bushings 2 enable rotational motion ofshafts 9 with respect to bushings 2, thereby providing a gimbaledmounting. End caps 1 and 13 are secured together by means of bolts 16and threaded inserts 18. Spring 6 provides electrical contact betweenshafts 9, which are electrically conductive and are electricallyconnected to output terminals (not shown) of the geophone, and bushings2, which are also electrically conductive, and which are electricallyconnected to the streamer cable wiring. Thrust washers 7 providepre-load for bearings 4 to eliminate undesired bearing slack. O-rings 15provide a seal between outer sleeve 12 and end caps 1 and 13, andO-rings 14 provide a seal between bushings 2 and end caps 1 and 13. Plug17 is utilized for plugging the conduit through which fluid is insertedinto the interior of the geophone housing comprising the two end caps 1and 13, and the outer sleeve 12. The configuration of the geophoneassembly illustrated in FIGS. 3 and 4 is a particular implementation ofan embodiment of the invention is not intended to be limiting. Thegeophone assembly 3 is secured to a seismic cable strain member forpositive location.

[0038] The housing 20, comprising end caps 1 and 13 and outer sleeve 12,contains a fluid, preferably an oil, which substantially surrounds thegeophone. The fluid provides coupling between the geophone and thegeophone housing of the geophone assembly. The fluid should preferablysurround the geophone, but preferably will not entirely fill the housingso as to allow room for fluid expansion and contraction with changes intemperature and pressure. The fluid has a viscosity that providessufficient damping of geophone movement to reduce noise, while enablingsufficient movement of the geophone 10 on the bearings to maintain thetransducer in the desired orientation. That is, the viscosity of thefluid should be high enough to restrain the geophone from unwantedmovements but low enough to prevent the geophone from followingrotational movement of the housing and a streamer in which the geophoneassembly may be mounted. A preferred viscosity for such fluid is in therange of about 500 to about 5000 centistokes.

[0039] A positioner, such as a weight 11, may be mounted on the lowerside of the geophone 10 to assist in maintaining the sensor 10 in thedesired orientation. Positioner 11 may be formed substantially fromlead, although other materials having a density greater than the densityof the geophone may be utilized. Alternatively, or additionally, apositioner (not shown) having a density lower than the density of thefluid that substantially surrounds the geophone 10, may be installed onthe upper side of geophone 10 to assist in maintaining the geophone 10in the desired orientation. Locating the center of gravity of thegeophone below the rotational axis of the gimbal on which the sensor ismounted will also assist in maintaining the geophone in the desiredorientation.

[0040] The particle velocity sensor in accordance with this invention issufficiently small to fit in the interior of a cylindrical streamercable. Typical internal diameters of such cylindrical streamer cablesare either 55 millimeters or 66 millimeters. The space within thestreamer surrounding the seismic sensors and other apparatus (not shown)positioned within the streamer is typically filled with a liquid, suchas an oil, which provides substantially neutral buoyancy to the cable.The space may also be filled with a gel or semi-solid material, and thestreamer may also be a solid streamer.

[0041] In a preferred embodiment of the invention, the density of theoverall geophone assembly (including the fluid and all other elementsthereof) is selected to improve coupling between the geophone assemblyand its surroundings. In general, optimum coupling is obtained when theacoustic impedance of the geophone assembly is about the same as theacoustic impedance of its surroundings, which may be achieved by makingthe density of the geophone assembly about the same as the density ofits surroundings, and the acoustic velocity of the geophone assemblyabout the same as the acoustic velocity of its surroundings.

[0042] When an acoustic wave traveling in one medium encounters theboundary of a second medium, reflected and transmitted waves aregenerated. Further, when the boundary area of the second medium is muchsmaller than the wavelength of the acoustic wave, diffraction resultsrather than reflection. For plane waves the characteristic acousticimpedance of a medium is equal to density times velocity, i.e.,

z=±ρ ₀ c  (Eq. 1)

[0043] in which,

[0044] z=acoustic impedance

[0045] ρ₀=density, and

[0046] c=velocity.

[0047] Let the incident and reflected wave travel in a fluid ofcharacteristic acoustic impedance, r₁=ρ₁c₁, where ρ₁ is equilibriumdensity of the fluid and c₁ is the phase speed in the fluid. Let thetransmitted wave travel in a fluid of characteristic acoustic impedancer₂=ρ₂c₂. If the complex pressure amplitude of the incident wave is P₁,that of the reflected wave P_(R), and that of the transmitted waveP_(T), then the pressure reflection coefficient R may be defined as:$\begin{matrix}{{R = \frac{r_{2} - r_{1}}{r_{2} + r_{1}}},} & \left( {{Eq}.\quad 2} \right)\end{matrix}$

[0048] and since 1+R=T, the pressure transmission coefficient T can bewritten as: $\begin{matrix}{T = \frac{2r_{2}}{r_{2} + r_{1}}} & \left( {{Eq}.\quad 3} \right)\end{matrix}$

[0049] It follows from the foregoing explanation that improved receptionwill be achieved if the particle velocity sensor is made in such a waythat the density and speed in the sensor assembly, including its housingand other components, is similar to that of the surrounding fluid. Ifthey are equal, the reflection coefficient will be R=0 and thetransmission coefficient will be T=1.

[0050] By making the acoustic velocity in the particle velocity sensorsubstantially equal to the acoustic velocity in the water in which thesensor is deployed, and by making the density of the particle velocitysensor similar to the density of the water, a good impedance match isgenerated between the water and the particle velocity sensor. Thevelocity sensor will have a good impedance match with the surroundingmedia and no distortion of amplitude or phase will occur due toreflection, diffraction or other anomalies of the traveling wave passingthrough the sensor and its housing.

[0051] In a preferred embodiment, the density of the particle velocitysensor is less that about twice the density of water (about 2 g/cm³),and more preferably about the same as the density of water (about 1g/cm³). Accordingly, the density of the particle velocity sensor shouldtypically be between about 0.5 g/cm³ and 2 g/cm³, and more preferably,about 1.0 g/cm³. It is understood, however, that water density may varywith salinity, and that it may be useful to vary the density of theparticle velocity sensor, depending on the particular body of water inwhich the particle velocity sensor is to be employed. Because thedensity of particle velocity sensor in accordance with a preferredembodiment of the invention is substantially less than the density ofgeophone assemblies typically available for use in ocean bottom seismicoperations, different components are selected from which to assemble thegeophone assembly. For example, at least a portion of the housing may beformed from a moldable elastomeric, such as isoplast or polypropylene,or a moldable composite material, such as fiber reinforced epoxy.

[0052] Over and above the need for good acoustic coupling, a low-weightparticle velocity sensor is useful because, in a preferred embodiment ofthe invention, the seismic cable in which the sensors are included needsto be neutrally buoyant. As many as 10,000 particle velocity sensors maybe utilized in a single cable. Accordingly, a particle velocity sensorhaving a density of less than 2 grams per cubic centimeter facilitatesthe mechanical construction of the seismic cable to achieve neutralbuoyancy.

[0053] In a particular implementation of the invention, particlevelocity sensors 3 and pressure gradient sensors 5 are utilized togetherin a cylindrical seismic cable 30, as shown in FIG. 5. Use of bothparticle velocity sensors and pressure gradient sensors enables signaldegradation resulting from surface ghost reflections to be substantiallyeliminated from the recorded seismic data. Such signal improvement isachieved by combining the output signals from a particle velocity sensor(or an array of particle velocity sensors) with the output signal from apressure gradient sensor (or an array of pressure gradient sensors)positioned at substantially the same location. Particle velocity sensorsand pressure gradient sensors positioned at substantially the samelocation may be referred to hereinafter as “co-located” sensors.

[0054] The phase and amplitude response for a pressure gradient sensorare substantially constant in the seismic frequency band of interest(from about 2 Hz. to about 300 Hz.). For example, for the T-2BXhydrophone marketed by Teledyne Instruments, Inc. of 5825 Chimney RockRoad, Houston, Tex. 77081, the variation in amplitude over a frequencyrange of 2-300 Hz. has been measured at less than 1 db, and thevariation in phase at less than 0.1 degree. FIGS. 6A and 6B show atypical amplitude and phase response for a particle velocity sensor. InFIG. 6A, curve 56 represents amplitude variation, and in FIG. 6B, curve58 represents the phase variation. In contrast to the amplitude andphase response of the hydrophone, it is evident that there aresubstantial variations in both the amplitude and the phase response fora particle velocity sensor in the seismic frequency range of interest.

[0055] Further, in prior art systems, in which the impedance of theparticle velocity sensor was not substantially matched to the impedanceof the substance (either the water or the water bottom) from which theseismic wave is coupled to the particle velocity sensor, additionalvariations in amplitude and phase occur in the seismic frequency rangebecause of the impedance mismatch.

[0056] In accordance with a particular embodiment of the presentinvention, where the impedance match between the water and the particlevelocity sensor is more nearly equal, such additional variations inamplitude and phase are minimized, and, accordingly, the particlevelocity sensor output and the pressure gradient sensor output can bematched by utilizing an appropriate filter, of a type known to those ofordinary skill in the art, without requiring additional matching forvariations caused by impedance mismatch.

[0057] In one implementation of the invention, the pressure gradientsensor is a hydrophone and the particle velocity sensor is a geophone.The ratio of acoustic pressure in a medium to the associated particlevelocity speed is the specific acoustic impedance (ρ₀c=p/u). For ahydrophone, having a good impedance match to the medium surrounding thehydrophone, and having (for example) a pressure sensitivity of 20 voltsper bar, i.e.,

H=20V/bar,  (Eq. 4)

[0058] which relationship may be expressed as

H=20V/10⁵ N/m ²,  (Eq. 5)

[0059] and a geophone or a group of geophones, having a good impedancematch to the medium surrounding the geophone, and having (for example) avoltage sensitivity of:

G=20V/m/s,  (Eq. 6)

[0060] the scale factor (K), expressing the relationship betweenvelocity output signal of the geophone and the pressure output signal ofthe hydrophone will be: $\begin{matrix}{K = {\frac{{H \cdot \rho_{0}}c}{G} = {\frac{20 \cdot {10^{- 5}\left\lbrack {V\text{/}N\text{/}m^{2}} \right\rbrack} \cdot 1.5 \cdot {10^{6}\left\lbrack {{Ns}\text{/}m^{3}} \right\rbrack}}{20\left\lbrack {V\text{/}m\text{/}s} \right\rbrack} = 15}}} & \left( {{Eq}.\quad 7} \right)\end{matrix}$

[0061] which indicates that the geophone velocity output signal needs tobe multiplied with a scale factor of K=15 before the pressure and thevelocity can be compared. It will be understood that for hydrophones andgeophones having different sensitivities than in the example discussedabove, the scale factor (K) will be different. Further, because of thevariation in the amplitude (as shown in FIG. 6A) and phase (as shown inFIG. 6B) of the geophone output as a function of frequency, it isnecessary to compensate for the amplitude and phase response of thegeophone before applying the scale factor.

[0062] The amplitude response (E) and phase response (φ) for thegeophone as a function of frequency may be represented by the followingrelationships: $\begin{matrix}{E = \frac{{G\left( \frac{f^{2}}{f_{n}^{2}} \right)}\left( \frac{R}{r + R} \right)}{\sqrt{\left( {1 - \frac{f^{2}}{f_{n}^{2}}} \right) + {4b_{t}^{2}\frac{f^{2}}{f_{n}^{2}}}}}} & \left( {{Eq}.\quad 8} \right) \\{\varphi = {\alpha \quad {\cot \left( \frac{2b_{1}\frac{f}{f_{n}}}{1 - \frac{f^{2}}{f_{n}^{2}}} \right)}}} & \left( {{Eq}.\quad 9} \right)\end{matrix}$

[0063] in which,

[0064] G=geophone voltage sensitivity;

[0065] f=frequency;

[0066] f_(n)=natural resonance frequency;

[0067] r=winding resistance;

[0068] R=load resistance; and

[0069] b_(t)=total damping.

[0070] Typical values may be: f_(n)=10; r=350 ohms; R=∞; and b_(t)=0.6.

[0071] If the amplitude and phase of the geophone output signal isadjusted to compensate for this variation in phase and amplitude withfrequency, the geophone output signal will have substantially the samephase and amplitude curve as the hydrophone signal. Normally theadjustment may be made on the basis of calculations based on Equations 8and 9.

[0072] As stated above, in a preferred embodiment of the invention,particle velocity sensors are constructed to have an acoustic impedancesubstantially similar to the acoustic impedance of the water in the bodyof water in which the particle velocity sensors are deployed.Accordingly, problems encountered in prior art system, in which theimpedance of the sensor was not matched to the acoustic impedance of themedium from which a seismic wave was coupled to the sensor, are avoided.In prior art systems variations in amplitude and phase as a function offrequency caused by impedance mismatch compounded the difficulty ofmatching the particle velocity sensor output to the pressure gradientsensor output. Because of the impedance match achieved in a preferredembodiment of the present invention, only the variation in amplitude andphase of the particle velocity sensor itself needs to be compensated forto enable the particle velocity sensor output to be combined with thepressure sensor output to attenuate the spectral notches caused by theghost reflection.

[0073] In a preferred embodiment of the invention, the phase andamplitude variations with frequency of the particle velocity sensor maybe calculated based on known (or determinable) characteristics of theparticle velocity sensor, itself. The output signal of the particlevelocity sensor may be modified accordingly to correct for amplitude andphase variation with frequency using filter techniques well known tothose of ordinary skill in the art. For co-located pressure gradientsensors and particle velocity sensors, the signal output of the pressuregradient sensor and the filtered output of the pressure gradient sensormay then be summed to attenuate the spectral notches resulting from theghost reflection. Although, in a preferred embodiment of the invention,the phase and amplitude of the particle velocity sensor output ismodified to substantially match the pressure gradient sensor output,those of ordinary skill in the art would understand that the phase andamplitude of the pressure gradient sensor output could be modified tomatch the particle velocity sensor output signal.

[0074] Because the noise level is generally greater at shallower waterdepths, placing the streamer at depths greater than about nine meters(the greatest depth at which streamer cables are typically deployed) mayreduce noise detected by the sensors, and the signal to noise ratio ofthe signals detected by the seismic sensors is accordingly improved.However, for such greater depths, notches in a hydrophone spectrumresulting from the surface ghost reflection are at lower frequencies,and such a hydrophone signal is normally regarded as undesirable becauseof the spectral notches in the frequency range of interest in seismicexploration. In accordance with an embodiment of the present invention,the output signal from the particle velocity sensor, which will havenotches in its frequency spectrum at different frequencies from thenotches in the frequency spectrum of the hydrophone, may be combinedwith the hydrophone output signal to compensates for the notches and asubstantially ghost free signal can be obtained. FIG. 7 shows simulatedoutput responses for a hydrophone (curve 42) and a geophone (curve 44)at a water depth of 26-meters. The graph indicates that two signals maybe combined to compensate for the spectral notches resulting from thesurface reflection. FIG. 8 provides actual data from a field test withthe cable at about 26 meters, which confirms the results indicated inthe simulation. In FIG. 8 the geophone output signal is designated bynumeral 46 and the hydrophone output signal is designated by numeral 48.FIG. 9 shows a summation (curve 60) of the hydrophone and geophone datashown in FIG. 8, and illustrates the attenuation of the spectral notches

[0075] Because of the potential high noise level in geophone signals atlow frequencies, resulting from mechanical vibrations in the cable, in aparticular implementation of the invention, low frequency geophonesignals are not combined with the hydrophone signal. In one specificimplementation of the invention, frequencies in the geophone signallower than about the frequency of the lowest frequency spectral notch inthe hydrophone spectrum are removed from the geophone signal before thegeophone signal is combined with the hydrophone signal. In anotherimplementation of the invention, geophone signals of less than about 30Hz. are not combined with the hydrophone signal.

[0076] Improved results are also afforded for operations at shallowdepths by the use of particle velocity sensors in seismic cables inaddition to pressure gradient sensors, over operations using solelypressure gradient sensors. At shallower depths, i.e., less than about 6meters, a hydrophone output signal will be attenuated by the surfaceghost in the seismic frequency range of interest. Because of the phasedifference between the upgoing pressure gradient wavefield and thedowngoing pressure gradient wavefield within the seismic frequency bandof interest, the downgoing wavefield is subtractive with respect to theupgoing wavefield and the downgoing wavefield effectively attenuates theupgoing wavefield. For a geophone signal, however, the result is theopposite, and the surface ghost signal effectively increases theamplitude of the signal detected by the geophone. The difference inphase between the upgoing wavefield and the downgoing wavefield is suchthat, for shallow depths, the signal detected by the geophone isadditive. Accordingly, substantially improved results are achieved byuse of particle velocity sensors in addition to pressure gradientsensors at shallow depths over what is achieved by use of pressuregradient sensors alone. In coastal regions where the water depth isquite shallow, it may be particularly useful to be able to deploy thesensors at such shallower depths.

[0077]FIG. 10 shows a simulation of a hydrophone signal (curve 52) and ageophone signal (curve 50) at one-meter depth. The attenuation of thehydrophone signal is evident. Combining the geophone output signal withthe hydrophone output signal for data recorded at the one-meter waterdepth also compensates for the influence from the surface reflection.

[0078] Generally, a hydrophone signal will have an amplitude that is 10to 20 times greater than the amplitude of a geophone signal. Thisrelationship will vary depending on the particular sensitivity of theparticular sensors used. Typically a group of hydrophones, distributedacross a selected spatial distance, will be connected in parallel fornoise attenuation, and the hydrophone output signal that is recorded foruse in seismic data processing and analysis is the combined output froma plurality of individual hydrophones connected in parallel. Because ofthe lower signal amplitude of the geophone output signal, in oneimplementation of the invention, a group of geophones, associated with agroup of hydrophones (co-located geophones and hydrophones), will beconnected in series, to increase the amplitude of the output signal aswell as to attenuate noise, and the geophone output signal that isrecorded for use in seismic data processing and analysis will be thecombined output from a plurality of individual geophones connected inseries. However, depending on the needs of a particular survey, thegeophone groups may be connected in parallel or series, or in aparallel/series combination. Although, in general, the discussion hereinrefers to an output signal from various sensors, the output signal istypically the output signal from a plurality of discrete sensorsinterconnected into a sensor array. Further, although the discussionherein generally refers to a geophone and hydrophone, particle velocitysensors other than geophones and pressure gradient sensors other thanhydrophones are intended to be within the scope of the presentinvention.

[0079] In one embodiment, groups of about 8 pressure gradient sensorswill be used in association with groups of about 2 to about 16 particlevelocity sensors (with lower numbers rather than higher numbers ofparticle velocity sensors preferred), and each combined group will beabout 12.5 meters apart from another such combined group. In thisembodiment, combined groups of both pressure sensors and particlevelocity sensors will be treated as single sensors.

[0080] In one embodiment of the invention, three-component particlevelocity sensors are included in the seismic cable. By “three-component”is meant that, in addition a particle velocity sensor (typically ageophone) mounted to sense motion in the vertical direction, twoparticle velocity sensor are mounted in orthogonal directions withrespect to each other (and to the vertically mounted geophone) to sensehorizontal motion. Accordingly, a three-component particle velocitysensor will sense motion in the vertical direction, in an in-linedirection and a cross line direction. Positioning these sensors in thesethree directions enables the propagation direction of an incoming signalto be detected, and also enables the detection of strumming or othermechanical behavior to the cable.

[0081] Accelerometers could be used as particle motion sensor as analternative to use of geophones, although the output signal will need tobe integrated to obtain velocity. An example commercial accelerometersuitable for use in the present invention is the VECTOR-SEIS™, availablefrom Input Output, Inc. in Houston, Tex. This particular accelerometergenerates a DC output signal which is indicative of the variation inorientation of the accelerometer from a selected orientation,accordingly, if sets of 2 (for situations in which the in-line directionis known) or 3 (if the in-line direction is not known) of theseaccelerometers are utilized, the sensor orientation may be computed andit is not necessary to gimbal-mount the accelerometers. A singleaccelerometer could also be used, but it would need to begimbal-mounted. Since the sensor can measure acceleration to DC, it ispossible to determine the true gravity vector by analyzing the magnitudeof G (the gravity vector) each sensor is operable under. The results ofthis analysis are stored with the trace data as direction cosines anddescribe the tensor rotation required to recover the signals as if thesensor were deployed at true vertical orientation.

[0082] The foregoing description of the invention is intended to be adescription of preferred embodiments. Various changes in the describedapparatus and method can be made without departing from the intendedscope of this invention as defined by the appended claims.

What is claimed is:
 1. A particle velocity sensor, comprising: ahousing; a geophone mounted in said housing; a fluid within said housingsubstantially surrounding said geophone; and wherein said particlevelocity sensor has an acoustic impedance within the range of about750,000 Newton seconds per cubic meter to about 3,000,000 Newton secondsper cubic meter.
 2. The particle velocity sensor of claim 1 wherein theacoustic impedance of said particle velocity sensor is substantiallyequal to the acoustic impedance of sea water.
 3. The particle velocitysensor of claim 1 wherein said particle velocity sensor has a density ofless than 2 grams per cubic centimeter. 4 The particle velocity sensorof claim 1 wherein said particle velocity sensor has a density equal toabout 1 gram per cubic centimeter.
 5. The particle velocity sensor ofclaim 1 wherein said particle velocity sensor has a densitysubstantially equal to the density of seawater.
 6. The particle velocitysensor of claim 1 wherein said geophone is gimbal-mounted within saidhousing and said fluid has a viscosity selected to restrainnoise-generating movement of said geophone and to allow said geophone tomaintain a selected orientation as said housing is rotated.
 7. Theparticle velocity sensor of claim 6 wherein said fluid has a viscositygreater than about 500 centistokes and less than about 5000 centistokes.8. The particle velocity sensor of claim 1 further comprising electricalconductors coupled to said geophone for conveying geophone outputsignals to the exterior of said housing, each electrical conductorcomprising an electrically conductive spring in electrical communicationwith said geophone.
 9. The particle velocity sensor of claim 1 whereinthe dimensions of said particle velocity sensor are adapted for mountingwithin the internal diameter of a seismic streamer cable
 10. Theparticle velocity sensor of claim 9 wherein the internal diameter ofsaid seismic streamer cable is about 55 millimeters.
 11. The particlevelocity sensor of claim 9 wherein the internal diameter of said seismicstreamer cable is about 66 millimeters.
 12. A particle velocity sensor,comprising: a housing; a geophone mounted in said housing; a fluidwithin said housing substantially surrounding said geophone; and whereinsaid particle velocity sensor has a density of less than 2 grams percubic centimeter.
 13. The particle velocity sensor of claim 12 whereinsaid particle velocity sensor has a density equal to about 1 gram percubic centimeter.
 14. The particle velocity sensor of claim 12 whereinsaid particle velocity sensor has a density substantially equal to thedensity of seawater.
 15. The particle velocity sensor of claim 12wherein said particle velocity sensor has an acoustic impedance withinthe range of about 750,000 Newton seconds per cubic meter to about3,000,000 Newton seconds per cubic meter.
 16. The particle velocitysensor of claim 12 wherein the acoustic impedance of said particlevelocity sensor is substantially equal to the acoustic impedance of seawater.
 17. The particle velocity sensor of claim 12 wherein saidgeophone is gimbal-mounted within said housing and said fluid has aviscosity selected to restrain noise-generating movement of saidgeophone and to allow said geophone to maintain a selected orientationas said housing is rotated.
 18. The particle velocity sensor of claim 17wherein said fluid has a viscosity greater than about 500 centistokesand less than about 5000 centistokes.
 19. The particle velocity sensorof claim 12 further comprising electrical conductors coupled to saidgeophone for conveying geophone output signals to the exterior of saidhousing, each electrical conductor comprising an electrically conductivespring in electrical communication with said geophone.
 20. The particlevelocity sensor of claim 12 wherein the dimensions of said particlevelocity sensor are adapted for mounting within the internal diameter ofa seismic streamer cable
 21. The particle velocity sensor of claim 20wherein the internal diameter of said seismic streamer cable is about 55millimeters.
 22. The particle velocity sensor of claim 20 wherein theinternal diameter of said seismic streamer cable is about 66millimeters.
 23. A particle velocity sensor, comprising: a housing; agimbal-mounted geophone mounted in said housing; a fluid within saidhousing substantially surrounding said geophone; and wherein said fluidhas a viscosity selected to restrain noise-generating movement of saidgeophone and to allow said geophone to maintain a selected orientationas said housing is rotate.
 24. The particle velocity sensor of claim 23wherein said fluid has a viscosity greater than about 500 centistokesand less than about 5000 centistokes.
 25. The particle velocity sensorof claim 23 wherein said particle velocity sensor has an acousticimpedance within the range of about 750,000 Newton seconds per cubicmeter to about 3,000,000 Newton seconds per cubic meter.
 26. Theparticle velocity sensor of claim 23 wherein the acoustic impedance ofsaid particle velocity sensor is substantially equal to the acousticimpedance of sea water.
 27. The particle velocity sensor of claim 23wherein said particle velocity sensor has a density of less than 2 gramsper cubic centimeter.
 28. The particle velocity sensor of claim 23wherein said particle velocity sensor has a density equal to about 1gram per cubic centimeter.
 29. The particle velocity sensor of claim 23wherein said particle velocity sensor has a density substantially equalto the density of seawater.
 30. The particle velocity sensor of claim 23further comprising electrical conductors coupled to said geophone forconveying geophone output signals to the exterior of said housing, eachelectrical conductor comprising an electrically conductive spring inelectrical communication with said geophone.
 31. The particle velocitysensor of claim 23 wherein the dimensions of said particle velocitysensor are adapted for mounting within the internal diameter of aseismic streamer cable.
 32. The particle velocity sensor of claim 31wherein the internal diameter of said seismic streamer cable is about 55millimeters.
 33. The particle velocity sensor of claim 31 wherein theinternal diameter of said seismic streamer cable is about 66millimeters.
 34. A particle velocity sensor, comprising: a housing; ageophone mounted in said housing; a fluid within said housingsubstantially surrounding said geophone; and wherein the dimensions ofsaid particle velocity sensor are adapted for mounting within theinternal diameter of a seismic streamer cable
 35. The particle velocitysensor of claim 34 wherein the internal diameter of said seismicstreamer cable is about 55 millimeters.
 36. The particle velocity sensorof claim 34 wherein the internal diameter of said seismic streamer cableis about 66 millimeters.
 37. The particle velocity sensor of claim 34wherein said particle velocity sensor has an acoustic impedance withinthe range of about 750,000 Newton seconds per cubic meter to about3,000,000 Newton seconds per cubic meter.
 38. The particle velocitysensor of claim 34 wherein the acoustic impedance of said particlevelocity sensor is substantially equal to the acoustic impedance of seawater.
 39. The particle velocity sensor of claim 34 wherein saidparticle velocity sensor has a density of less than 2 grams per cubiccentimeter.
 40. The particle velocity sensor of claim 34 wherein saidparticle velocity sensor has a density equal to about 1 gram per cubiccentimeter.
 41. The particle velocity sensor of claim 34 wherein saidparticle velocity sensor has a density substantially equal to thedensity of seawater.
 42. The particle velocity sensor of claim 34wherein said geophone is gimbal-mounted within said housing and saidfluid has a viscosity selected to restrain noise-generating movement ofsaid geophone and to allow said geophone to maintain a selectedorientation as said housing is rotated.
 43. The particle velocity sensorof claim 39 wherein said fluid has a viscosity greater than about 500centistokes and less than about 5000 centistokes.
 44. The particlevelocity sensor of claim 34 further comprising electrical conductorscoupled to said geophone for conveying geophone output signals to theexterior of said housing, each electrical conductor comprising anelectrically conductive spring in electrical communication with saidgeophone.
 45. A method of geophysical exploration comprising: generatinga seismic signal in a body of water; detecting said seismic signal witha plurality of co-located particle velocity sensors and pressuregradient sensors positioned within a seismic cable deployed in said bodyof water; modifying the output signal of at least one of said particlevelocity sensors or said pressure gradient sensors to substantiallyequalize the output signals from said particle velocity sensors and saidpressure gradient sensors within at least a selected frequency range;and combining the modified output signals from co-located particlevelocity sensors and pressure gradient sensors within at least saidselected frequency range.
 46. The method of claim 45 wherein theamplitude and phase of the output signals from said particle velocitysensors and said pressure gradient sensors are substantially matchedwithin said at least a selected frequency range
 47. The method of claim45 wherein modifying the output signals from either said particlevelocity sensors or said pressure gradient sensors is performedindependently of the acoustic impedance of material through which saidseismic signal travels.
 48. The method of claim 45 wherein the outputsignals of said pressure gradient sensors and said particle velocitysensors are substantially equalized during processing and combined. 49.The method of claim 45 wherein the amplitude and phase of the pressuregradient sensors and the particle velocity sensors are equalized. 50.The method of claim 45 wherein output signals from said particlevelocity sensor and sand pressure gradient sensor are combined to reducespectral notches above frequencies of about 20 Hz.
 51. The method ofclaim 45 wherein said particle velocity sensors and pressure gradientsensors are positioned in the interior of a seismic cable having aninside diameter of about 55 millimeters.
 52. The method of claim 45wherein said particle velocity sensors and pressure gradient sensors arepositioned in the interior of a seismic cable having an inside diameterof about 66 millimeters.
 53. The method of claim 45 wherein said seismiccable is deployed at a depth of less that six meters.
 54. The method ofclaim 45 wherein said seismic cable is deployed at a depth of greaterthan nine meters.
 55. The method of claim 45 wherein said particlevelocity sensors have an acoustic impedance with the range of about750,000 Newton seconds per cubic meter to about 3,000,000 Newton secondsper cubic meter.
 56. The method of claim 45 wherein said particlevelocity sensors have an acoustic impedance substantially equal to theacoustic impedance of the water in said body of water in which saidcable is deployed.
 57. The method of claim 45 wherein said seismic cableis a liquid-filled cable.
 58. The method of claim 45 wherein saidseismic cable is a gel-filled cable.
 59. The method of claim 45 whereinsaid seismic cable is a solid cable.
 60. The method of claim 45 whereinsaid cable is towed through said body of water.
 61. The method of claim45 wherein said cable is maintained at a substantially stationaryposition.
 62. The method of claim 45 wherein at least a portion of theparticle velocity sensors are electrically interconnected in groups togenerate a particle velocity output signals.
 63. The method of claim 62wherein at least a portion of the particle velocity sensors areelectrically interconnected in series in groups of at least threesensors.
 64. The method of claim 62 wherein at least a portion of theparticle velocity sensors are electrically interconnected in parallel.65. The method of claim 45 wherein particle velocity sensors includesensors mounted in said cable in an orientation to detect signals in thevertical direction, the cross line direction and in-line direction. 66.A method of geophysical exploration comprising: deploying a seismiccable in a body of water, said seismic cable having a plurality ofparticle velocity sensor assemblies included within said cable, saidparticle velocity sensor assemblies having an acoustic impedance withinthe range of about 750,000 Newton seconds per cubic meter to about3,000,000 Newton seconds per cubic meter; and utilizing said seismiccable for detecting seismic data signals.
 67. The method of claim 66wherein said particle velocity sensor assemblies have an acousticimpedance equal to about the impedance of the water in said body ofwater in which said seismic cable is deployed.
 68. A method ofgeophysical exploration comprising: deploying a seismic cable in a bodyof water, said seismic cable having a plurality of particle velocitysensor assemblies included within said cable, said particle velocitysensor assemblies having a density less than 2 grams per cubiccentimeter; and utilizing said seismic cable for detecting seismic datasignals.
 69. The method of claim 68 wherein the density of said particlevelocity sensors is substantially equal to the density of the water insaid body of water in which said seismic cable is deployed.
 70. A methodof geophysical exploration comprising: deploying a seismic cable in abody of water, said seismic cable having a plurality of particlevelocity sensor assemblies included within said cable; wherein saidparticle velocity sensors comprise a housing, a gimbal-mounted geophonemounted in said housing, a fluid within said housing substantiallysurrounding said geophone, said fluid having a viscosity selected torestrain noise-generating movement of said geophone and to allow saidgeophone to maintain a selected orientation as said housing is rotate;and utilizing said seismic cable for detecting seismic data signals. 71.The method of claim 70 wherein said fluid has a viscosity of greaterthan about 500 centistokes and less than about 5000 centistokes.
 72. Amethod of processing marine seismic data to reduce spectral notchesresulting from surface ghost reflections, comprising: determining theamplitude and phase variation with frequency of the output of a particlevelocity sensor of a co-located sensor pair comprising a particlevelocity sensor and a pressure gradient sensor, independently of anyvariation in amplitude or phase with frequency of said particle velocitysensor resulting from impedance mismatch between said particle velocitysensor and a medium from which a seismic wave is coupled to saidparticle velocity sensor; modifying the output signal of at least one ofsaid particle velocity sensors or pressure gradient sensors tocompensate for said determined amplitude and phase variation, therebygenerating modified output signals; and summing said modified outputsignals from said pressure gradient sensor and said particle velocitysensor.